FIGS. 1A and 1B illustrate a method of allocating bursts in a conventional orthogonal frequency-division multiple access (OFDMA) system. That is, FIG. 1A illustrates a time division duplexing (TDD) frame used by the conventional OFDMA system according to IEEE 802.16e worldwide interoperability for microwave access (WiMAX). FIG. 1 B illustrates the method of allocating bursts in the conventional OFDMA system of IEEE 802.16e WiMAX. Referring to FIG. 1A, the TDD frame includes a downlink subframe and an uplink subframe. The horizontal axis represents an OFDMA symbol number and the vertical axis represents a subchannel number.
Referring to FIGS. 1A and 1B, in downlink, the method allocates bursts in a rectangular shape defined by a start point and an end point. In uplink, the method allocates bursts by allocating slots, each of which is a smallest allocation unit defined by a subchannel and an OFDMA symbol, to one subchannel along a time axis until a last symbol is allocated, and then by continuously allocating slots to a next subchannel at a first symbol along the time axis. The method has the following problems.
1) Since a start point and an end point of a burst should be defined in downlink, the amount of overhead transmitted to each downlink-MAP information element (DL-MAP IE) is increased.
2) If start points of all bursts in an uplink subframe are the same, when a distance between a transmitter and a receiver is long and propagation latency is big, in case of a terminal receiving a last burst in a downlink subframe, a predefined transmit-receive transition gap (TTG) is not long enough to shift from a reception mode to a transmission mode, thereby making it difficult to, transmit bursts in the same frame in uplink.
3) If end points of all the bursts in the uplink subframe are the same, it is difficult for a terminal requiring a quick response to the bursts transmitted in uplink to receive bursts of a next frame in downlink from a base station.